The protective effect of leucine on warm (37 C) global myocardial ischemia was evaluated in an isolated working heart model using Sprague-Dawley rats. Hemodynamic variables were measured before and after a 45 minute ischemic period. Experimental hearts were perfused with solutions of (L)-leucine at various concentrations in 0.9% saline. Control hearts were perfused with 0.9% saline alone. The optimum protection was afforded the hearts by 1mM leucine, which demonstrated significantly better protection than did the control solution. Doses of leucine at higher and lower concentrations, however, resulted in worse postischemic recovery of function as compared to controls.